1. Field of the Invention
The present invention relates to a circuit for controlling RF PAs (Radio Frequency Power Amplifiers), and more specifically, to an RF PA controller circuit that adjusts the supply voltage of RF PAs.
2. Description of the Related Art
RF (Radio Frequency) transmitters and RF power amplifiers are widely used in portable electronic devices such as cellular phones, laptop computers, and other electronic devices. RF transmitters and RF power amplifiers are used in these devices to amplify and transmit the RF signals remotely. RF PAs are one of the most significant sources of power consumption in these electronic devices, and their efficiency has a significant impact on the battery life of these portable electronic devices. For example, cellular telephone makers make great efforts to increase the efficiency of the RF PA systems, because the efficiency of the RF PAs is one of the most critical factors determining the battery life of the cellular telephone and its talk time.
Figure (FIG.) 1 illustrates a conventional RF PA system. The conventional RF PA system includes a transmitter integrated circuit (TXIC) 102 and an external power amplifier (PA) 104. In some cases, there may be a filter between the TXIC 102 and the PA 104. For example, the RF transmitter circuit may be included in a cellular telephone device using one or more cellular telephone standards (modulation techniques) such as UMTS (Universal Mobile Telephony System) or CDMA (Code Division Multiple Access), although the RF transmitter circuit may be included in any other type of RF electronic devices. For purposes of illustration only, the RF transmitter circuit will be described herein as a part of a cellular telephone device. The TXIC 102 generates the RF signal 106 to be amplified by the PA 104 and transmitted 110 remotely by an antenna (not shown). For example, the RF signal 106 may be an RF signal modulated by the TXIC 102 according to the UMTS or CDMA standard.
The RF power amplifier 104 in general includes an output transistor (not shown) as its last amplification stage. When an RF modulated signal 106 is amplified by the PA 104, the output transistor tends to distort the RF modulated signal 106, resulting in a wider spectral occupancy at the output signal 110 than at the input signal 106. Since the RF spectrum is shared amongst users of the cellular telephone, a wide spectral occupancy is undesirable. Therefore, cellular telephone standards typically regulate the amount of acceptable distortion, thereby requiring that the output transistor fulfill high linearity requirements. In this regard, when the RF input signal 106 is amplitude-modulated, the output transistor of the PA 104 needs to be biased in such a way that it remains linear at the peak power transmitted. This typically results in power being wasted during the off-peak of the amplitude of the RF input signal 106, as the biasing remains fixed for the acceptable distortion at the peak power level.
Certain RF modulation techniques have evolved to require even more spectral efficiency, and thereby forcing the PA 104 to sacrifice more efficiency. For instance, while the efficiency at peak power of an output transistor of the PA 104 can be above 60%, when a modulation format such as WCDMA is used, with certain types of coding, the efficiency of the PA 104 falls to below 30%. This change in performance is due to the fact that the RF transistor(s) in the PA 104 is maintained at an almost fixed bias during the off-peak of the amplitude of the RF input signal 106.
Certain conventional techniques exist to provide efficiency gains in the PA 104. One conventional technique is EER (Envelope Elimination and Restoration). The EER technique applies the amplitude signal (not shown in FIG. 1) and the phase signal (not shown in FIG. 1) of the RF input signal 106 separately to 2 ports of the power amplifier 104, i.e., its supply voltage port (Vcc) 108 and its RF input port 107, respectively. However, the EER technique often fails to provide significant efficiency gains, because the supply voltage 108 cannot be varied in an energy-efficient way to accommodate the large variations in the amplitude signal of the RF input signal 106; and thus, it fails to provide a substantial energy efficiency gain while maintaining the required linear amplification of the RF signal in the PA 104. This is mainly due to the difficulty in realizing a fast, accurate, wide range, and energy efficient voltage converter to drive the supply voltage of the PA 104.
The conventional EER technique can function better only if a variable power supply with a very large variation range is used to adjust the supply voltage based on the amplitude signal of the RF input signal 106, while not reducing the efficiency of the RF transmitter by the power consumed by the power supply itself. However, the variable power supply, which is typically comprised of a linear regulator (not shown in FIG. 1) that varies its output voltage on a fixed current load such as the PA 104 in linear mode, by principle reduces the supply voltage at constant current and by itself consumes the power resulting from its current multiplied by the voltage drop across the linear regulator when there is a large drop in the amplitude signal of the RF input signal 106. This results in no change in the overall battery power being consumed by the RF transmitter because any efficiency gained in the PA 104 is mostly lost in the linear regulator itself.
Variations of the EER technique, such as Envelope Following and other various types of polar modulation methods, likewise fails to result in any significant gain in efficiency in the RF transmitter, because the supply voltage is likewise adjusted based on the amplitude signal of the RF input signal 106 which inherently has large variations; and thus, has the same deficiencies as described above with respect to conventional EER techniques.
Some other conventional techniques improve the efficiency in the PA 104 by lowering the supply voltage 108 to the PA 104. In many of these techniques, both amplitude and phase components of the RF input signal 106 are fed to the PA 104 in a conventional manner. By using a lower supply voltage 108, the PA 104 operates with increased power efficiency because it operates closer to the saturation point. However, the supply voltage 108 cannot be reduced too low, because this would cause the PA 104 to operate with insufficient voltage headroom, resulting in unacceptable distortion. As described previously, the distortion may cause energy from the transmitted signal to spill over to adjacent channels, increasing spectral occupancy and causing interference to radios operating in those neighboring channels. Thus, an optimal supply voltage can be chosen for the PA which balances acceptable distortion with good efficiency.
One conventional method uses a step-down switched mode power supply (SMPS) (i.e., buck regulator) to lower the supply voltage 108 to the PA 104. However, choosing a fixed power supply voltage is not sufficient in many applications. For example, in most cellular systems, the PA output power changes frequently because the basestation commands the cellular handset to adjust its transmitted power to improve network performance, or because the handset changes its transmitted information rate. When the PA output power changes, the optimum supply voltage for the PA (as described above) changes.
Therefore, in some systems, the expected power of the RF output signal 110 is first determined, and then the power supply voltage 108 is adjusted in accordance with the expected power. By adaptively adjusting the supply voltage 108, the efficiency of the PA 104 is increased across various PA output power levels. Conventional methods estimate the expected power of the RF output signal 110 in an “open loop” manner, in which the power of the RF output signal 110 is estimated from the power of the delivered RF input signal 106. This method does not yield an accurate estimate of the power of the RF output signal 110 because the estimated power may vary according to various operating conditions, such as temperature, frequency, and mismatch. Therefore, even if the estimated power at some point approximates the actual power, changes in operating conditions result in deviation of the estimated power from the actual power.
Moreover, an estimate of the power of the RF output signal 110 may not be sufficient for properly adjusting the supply voltage 108. For example, the peak-to-average ratio (PAR) needs to be known in order to estimate the optimum supply voltage for the PA. The PAR refers to the difference of the mean amplitude and the peak amplitude in the modulated RF output signal 110. With a higher PAR, a higher supply voltage is needed to accommodate the peak voltage swings of the RF output signal 110. Many modern cellular systems change the PAR of the modulation in real time, requiring the supply voltage to be adjusted accordingly. Therefore, the conventional method of adjusting the supply voltage 108 of PA 104 based on an estimate of the PA output power is unsuitable in these cellular systems.
Further, the load presented to the PA 104 poses another problem. The PA 104 normally drives circuitry usually consisting of a filter and an antenna. Such circuitry often has an impedance around the range of 50 ohms. The impedance of the circuitry can sometime change radically. For example, if the antenna is touched or the cellular device is laid down on a metal surface, the impedance of the circuitry changes. The changes in the impedance of the circuitry coupled to the PA 104 may require changes in the supply voltage to the PA 104 to prevent distortion of the RF output signal 110 fed to this circuitry. The conventional methods described above, however, do not adjust the supply voltage in response to changes in the impedance of the circuitry.
Although the problems of inaccurate estimation of power at the RF output signal 110, changing PAR, and impedance changes at the output of PA 104 can be avoided by constantly providing a higher than optimum supply voltage to the PA 104, the higher supply voltage leads to a less efficient PA 104.
Thus, there is a need for a PA system that is efficient over a wide variety of modulation techniques and results in a significant net increase in power efficiency of the PA system. Additionally, there is a need for a PA controller that can adjust the power supply to the PA under conditions of varying temperature, frequency, output power, PAR, and impedance to maximize the PA efficiency while keeping distortion to an acceptable level.